The present invention relates to flow meters and in particular to flow meters operating on heat transfer principles.
There are many devices available for measuring fluid flow. It has been well known for centuries that increased fluid flow increases the rate of cooling of a hot object disposed in a flow stream. Several devices have been proposed for measuring flow, which make use of this principle, known as anemometry. The following U.S. patents disclose such devices:
U.S. Pat. No. 5,617,870, issued to Hastings and Feld
U.S. Pat. No. 5,582,628, issued to Wood X
U.S. Pat. No. 5,508,424, issued to Al-Ali
U.S. Pat. No. 5,373,850, issued to Kohno
U.S. Pat. No. 5,207,227, issued to Powers
U.S. Pat. No. 5,174,299, issued to Nelson
U.S. Pat. No. 4,677,985, issued to Bro and Carter
U.S. Pat. No. 4,354,504, issued to Bro
U.S. Pat. No. 3,789,831, issued to Kopaniky and Gann
U.S. Pat. No. 3,620,207, issued to Sinclair
In these devices, heat is typically provided with a resistance element. Some of these devices are proported to be minimally intrusive and are proposed for use in measuring blood flow in large veins and arteries. It is known that optical fibers with very small diameters are available and it is also known that thermocouple wire can be made of very small diameter material.
Resistive heaters used in prior art anemometers are larger than about 1 mm. Because of their size, the prior art flow probes cannot be used in small fluid pipes, such as small blood vessels. The relatively large size of the prior art probes does not allow enclosing those into a needle for measuring blood flow through capillary tissue or through porous media. Furthermore, for measuring flow of combustible or explosive fluids, such as underground oil or gas, it is highly undesirable to use electric heating for safety reasons.
What is needed is a less intrusive and safer than prior art flow measuring device.
The present invention provides a fluid flow monitoring device. A laser source produces a laser beam having a beam frequency that is transmitted through an optical fiber and absorbed in a small quantity of thermally conductive material having high optical absorption at the beam frequency. A temperature sensor is embedded in the light absorbing material. The light absorbing material is placed in a fluid flow and the light absorbing material is heated with energy in the laser beam. Voltage produced by the temperature sensor is monitored as an indication of the fluid flow. The flow-measuring device can be made extremely unintrusive using very thin temperature sensor wire and very thin optical fiber, both of which are contained within a very thin probe. The device is calibrated under known flow conditions. The flowing fluid cools the material. Using the calibration information, speed of the flowing fluid is determined by measuring the voltage produced by the temperature sensor.